1. Field of the Invention
The present invention relates to a light-scattering film and an optical device using the same.
2. Description of the Related Art
One of the devices sustaining the modern life is an optical device that realizes desired functions by utilizing the mutual conversion between electricity and light. A photoelectric conversion device (for example, a solar cell), a light-emitting device (for example, an electroluminescent element exemplified by a light-emitting diode and an OLED (organic light emitting diode)), and a liquid crystal element (for example, a liquid crystal display panel), are typical optical devices. Such optical devices are indispensable to the modern daily life.
An optical device includes a conductive structure member for guiding an electrical signal (for example, an electric current and a voltage) to a desired position, and a structure member for scattering light. For example, Japanese Laid Open Patent Application JP-A-Heisei, 6-313890 discloses a back electrode plate for a liquid crystal display device, provided with a metal reflective layer, an insulating light-scattering layer formed thereon, and a transparent electrode for coating the light-scattering layer. Further, Japanese Laid Open Patent Application JP-A-Heisei, 11-323196 discloses a reflective liquid crystal display device provided with a light-scattering layer in which a transparent resin is mixed with a light scatterer (see FIG. 1). In the reflective liquid crystal display device, the transparent electrode to which the electrical signal is applied, and the light-scattering layer are prepared separately. The Japanese Laid Open Patent Application JP-A-Heisei, 11-323196 further discloses that a light-scattering effect of the light-scattering layer can be improved by mixing spacer grains having a refractive index close to the transparent resin in order to provide a distance between transparent grains, which are the light scatterers (for example, the paragraph [0011]). In addition, Japanese Laid Open Patent Application JP-P2004-271600A discloses optical material in which the scatterers are distributed randomly, having an isotropic photonic gap with a large energy width that is less subjective to non-uniformity of the scatterers and a position deviation of the scatterers, and being capable of making a light guide and cavity of any shape.
To simplify the configuration of the optical device, it is preferable that the both functions of leading the electrical signal to a desired position and of scattering the light, are realized by a single structure member. One of such structures is a transparent electrode formed in a textured form (that is, with concavities and convexities), as shown in Japanese Laid Open Patent Application JP-P2004-271600A, Japanese Patent 2862174, Japanese Laid Open Patent Application JP-P2003-243676A. In Patent Documents 3 to 5, the transparent electrode formed in the textured form is used as an electrode on the side of a substrate of a photoelectric conversion device. Usage of the transparent electrode formed in the textured form, as the electrode on the side of the substrate, is one of the effective techniques to improve the conversion efficiency of the photoelectric conversion device. The transparent electrode formed in the textured form scatters incident light directed to the photoelectric conversion device, and effectively improves a light absorption amount, namely, the conversion efficiency. Further, in Japanese Laid Open Patent Application JP-P2002-222975A, a technique for resolving the trade-off between the advantages of optical and electrical characteristics by using the textured form conductive material is disclosed.
As methods to form a transparent electrode of the textured form, the following three methods are known. According to a first method, the transparent electrode is formed by using a thermal CVD (Chemical Vapor Deposition) method, as disclosed in Japanese Laid Open Patent Application JP-A-Heisei, 6-313890. By optimizing growth conditions, the transparent electrode of the textured form can be formed by using the thermal CVD method. According to a second method disclosed in Japanese Laid Open Patent Application JP-P2004-271600A, a surface of a glass substrate is polished and a transparent electrode is formed on the polished surface. A third method disclosed in Japanese Laid Open Patent Application JP-A-Heisei 11-323196 is a method by which a thin film is formed by insulating microparticles and binders on the substrate, and the transparent electrode is formed on the thin film.
However, undesired effect may also be generated if the concavities and convexities are provided to a conductive material for the purpose of scattering the light. For example, in the photoelectric conversion device, the usage of the transparent electrode formed in the textured form, as the electrode on the side of the substrate, induces defects to a semiconductor thin film formed thereon. This indicates that the improvement of the conversion efficiency of the photoelectric conversion device is limited, in the technique using the transparent electrode formed in the textured form as the electrode on the side of the substrate (see Yoshiyuki Nasuno et al., “Effects of Substrate Surface Morphology on Microcrystalline Silicon Solar Cells”, Jpn. J. Appl. Phys., The Japan Society of Applied Physics, 1 Apr. 2001, vol 40, pp. L303-L305.). If the concavities and convexities of the transparent electrode are enhanced, the light absorption of a semiconductor layer can be increased. However, the enhancement in the concavities and convexities of the transparent electrode increases the defects induced to the semiconductor thin film, and decreases an output voltage. Therefore, there is a limit to the improvement of the conversion efficiency realized by forming the concavities and convexities to the transparent electrode.
As a result, it is required to provide a technique for providing both the functions of guiding the electrical signal to the desired position and of scattering the light, with a single structure member with less concavities and convexities on a surface (the surface ideally is flat). Provision of such technique will also be effective to improve the conversion efficiency of the photoelectric conversion device, for example.
According to the present invention, it is possible to provide a technique for providing both functions of leading an electrical signal to a desired position and of scattering light, with a single structure with less concavities and convexities on a surface (the surface ideally is flat).
Furthermore, it is possible to further improve the conversion efficiency of a photoelectric conversion device by applying the present invention to the photoelectric conversion device.